1. Field of the Invention
The present invention generally concerns a process for producing a rutilized product from titaniferous material having a high titanium dioxide content and, more particularly, pertains to a process for retaining trivalent titanium values within the slag to permit selective chlorination of impurities in a fluid bed.
2. Description of the Prior Art
In order to obtain high grade titanium dioxide, which is widely used in the process for producing pigment in the paint industry, from titaniferous ores such as ilmenities, it is necessary to remove a substantial portion of the iron values therefrom. in commercial operations, partial removal of iron values is usually achieved from a thermo-reduction or electro-smelting treatment of the titaniferous ore which reduces the iron to a metallic state thus making it easily removable from the titanium values. After the iron is removed, a concentrate containing titanium values is recovered, this concentrate being generally referred to in the industry as titaniferous slag. Typically, this slag contains approximately 80% by wt. of titannium values, 10% by wt. of iron values and less than 2 wt.% of manganese values. Additionally, the slag contains impurities such as iron, manganese, magnesium, aluminum, silicon, calcium, chromium, and vanadium, and also contains titanium dioxide and reduced titanium values, proportions of each being dependent upon the starting ore material.
The term "reduced titanium values" as used herein is synonymous with low valent titanium values and is definitive of oxidic titanium compounds and complex compositions in which the titanium values are present in the material in the trivalent or divalent state.
Many methods have been utilized to produce an even higher content titanium dioxide concentrates from the produced titaniferous slag. As is well known, a fluid bed chlorination process is one means to selectively separate the remaining iron values from the titanium values contained within the slag. However, most such chlorination methods have been plagued by the formation of ferrous chloride which, at the temperatures used, becomes a stcky paste-like condensate which clogs up the reaction bed, conduits, values and other elements of the chlorination process, thereby inhibiting the efficiency thereof. Although some commercial processes have systems which can accommodate the formation of ferrous chloride, many commercial chlorination processes cannot handle this formation and are consequently limited to chlorinating either natural or synthetic rutiles or are restricted in their use of titaniferous slag to be only a relatively small fraction of the feed to the chlorinating bed.
A proposed solution to this problem of the production of ferrous chloride is to selectively chlorinate the pre-reduced ilmenite in the presence of carbon or other carbonaceous material. As disclosed in U.S. Pat. No. 2,852,362, carbon is used in the fluid bed chlorination process to maintain the fluid bed in a substantially iron-free state. The carbonaceous material competes for the oxygen of the ferrous oxide and in so doing, the iron, during the chlorination process, more easily combines with the chlorine to produce the volatile ferric chloride which is relatively easy to remove from the fluid bed. Such formation and removal of volatile ferric chloride can be accomplished without the forfeiture of titanium values.
However, important drawbacks to this solution exist. Ilmenites, by definition, contain large amounts of iron and, consequently, require large amounts of carbonaceous material to prevent the formation of ferrous chloride. Accordingly, it has been found desirable to provide a process which removes the necessity of the use of carbonaceous material yet inhibits the formation of ferrous chloride while maintaining the desired content of the titanium values within the product.
U.S. Pat. No. 2,747,987 describes a process to upgrade the titanium within the slag by the selective chlorination of the iron values within a fluid bed to form ferric chloride. The reduced titanium content maintained in the slag is in an amount sufficient to accept substantially all of the oxygen from the iron values in the slag. This allows the chlorination process to proceed without the necessity for the presence of any carbonaceous reducing agent. The basic formula for this reaction is: EQU FeO+Ti.sub.2 O.sub.3 +Cl.sub.2 .fwdarw.FeCl.sub.2 +2TiO.sub.2
The ferrous oxide reacts with the reduced titanium and chlorine gas to produce ferrous chloride and titanium dioxide. Ferrous chloride is then further partially converted by the reaction with chlorine to form ferric chloride.
However, nothwithstanding the fact that the chlorination reaction is strongly exothermic, the process described by this patent is not autogenous and, therefore, essentially 80% to 90% of the titaniferous slag does not take part in the above-described reaction and, consequently, must be discharged from the fluid bed reactor at the reaction temperature, which is 900.degree.-1000.degree. C. Therefore, this subsequent heat loss has to be compensated by preheating the slag.
The inability of the reaction to sustain itself is complicated by the fact that it is also well known that titaniferous material is highly sensitive to oxygen. To merely heat the slag, prior to its introduction, is known to decrease the amount of reduced titanium available within the slag. U.S. Pat. No. 2,715,501 discloses that heating a titaniferous slag in air at 200.degree. C. for one hour oxidized 19.1% of the reduced titanium. This would effectively remove this reduced titanium from becoming an acceptor of oxygen from the ferrous oxide. U.S. Pat. No. 3,868,441 further discloses that the oxidation of slag proceeds in several stages, with the oxidaton of all reduced titanium occuring at about 450.degree. C.